Wind turbine blades and thereby also the root region thereof are often made by assembling two blade halves essentially corresponding to the suction side and the pressure side, respectively, along the chord plane. However, the blades may also be moulded in their entirety by so-called hollow moulding.
The root region of a known blade comprises layers of fibres forming an outer layer and an inner layer between which fastening members in the form of bushings are placed. A separately formed inserts may be placed between each pair of adjacent bushings, whereby the bushings are mutually separated by the inserts. The known inserts are made of glass fibres embedded in a suitable resin.
A potential problem in connection with wind turbine blades is load transfer from the fibre composite structure of the root region to the hub of the wind turbine. The connection and transfer of loads from the blade to the hub is inter alia provided by mounting the blade to the hub by screwing bolts into the bushings placed in the root or by means of nuts screwed onto stud bolts screwed into the bushings. In case the number of bolts and thereby the number of bushings has to be increased to handle a given load, remaining area of the fibre composite material between the bushings is reduced. This may result in the root connection being insufficiently supported to withstand the loads, whereby the connection between the blade root and the hub may fail since the bushings are insufficiently retained in the composite material and thus pulled out of the composite material of the root region. This is especially a problem when long and thereby heavy blades are to be used.
WO 2010/018225 provides a method of manufacturing a wind turbine blade comprising a steel wire or steel fibre-reinforced polymer matrix. However, the document does not address the problem of how the root region is to be designed to withstand extreme loads in the connection between the blade root and the hub.
GB 2 472 460 discloses a wind turbine blade with a connection piece having a u-shaped section for fitting to sides of a root laminate. The connection piece presents an alternative to bonded or laminated-in bushings.
DE 196 25 426 discloses a rock anchor comprising a core element made of polymer and provided with outer threads. The outer part of the rock anchor is reinforced with glass fibres. The anchor is particularly suited for non-conductive, non-magnetic and dielectric parts.
WO 03/057457 discloses a wind turbine blade comprising root end bushings with wedges provided in extension of the bushings.
WO 2011/035538 discloses an insert for a wind turbine blade root. The insert comprises a flat wedge-like portion, which is laminated into the root laminate, and a connection portion for fitting the insert to a mount. The connection portion is located outside the root laminate.